1. Field of the Invention
One or more embodiment of the present general inventive concept relates to encoding or decoding an audio signal, and more particularly, to a method and apparatus to encode or decode a high frequency signal contained in a band of frequencies which is greater than a predetermined frequency.
2. Description of the Related Art
Audio signals, such as speech signals or music signals, can be divided into low frequency signals contained in a band of frequencies that is less than a predetermined frequency and high frequency signals contained in a band of frequencies that is greater than the predetermined frequency. Since high frequency signals are less important in human sound perception than low frequency signals due to human hearing characteristics, generally, a small number of bits are allocated to high frequency signals when encoding an audio signal. Spectral Band Replication (SBR) is an example of a technique of encoding/decoding an audio signal using this concept. In SBR, an encoder encodes a high frequency signal by using a low frequency signal, and a decoder decodes the encoded high frequency signal by using a decoded low-frequency signal. However, when a high frequency signal is produced by simply replicating a low frequency signal and then decoded as in the conventional art, a high frequency signal obtained by the decoding differs from the high frequency signal of the original signal, and thus sound quality is greatly diminished.
Traditionally, a difference between the characteristics of the original high-frequency signal and a restored high-frequency signal is compensated using an adaptive whitening filter or a noise-floor. When the high frequency signal to be restored is tonal, but has a strong inclination toward noise, an adaptive whitening filter changes the inclination of the high frequency signal toward noise by using an inverse-filtering process. By using a noise-floor, noise is added to the high frequency signal to reduce a difference between tonalities of a high frequency signal to be restored and the original high-frequency signal.